1. Field of the Invention
This invention relates to a method and apparatus for cleaning particulate materials such as soils which are contaminated with a variety of contaminants such as heavy metals, radioactive compounds and organics, often in combination, through a combination of leaching, washing, attrition scrubbing, countercurrent flow size separation and density separation. This invention further relates to the recovery of such contaminants following removal from the soils, for additional processing, recycling or disposal.
2. Background Information
Contaminated soil is becoming a more common environmental problem. The contaminants can include heavy metals, such as for instance, copper, lead and mercury; radioactive species such as for example, radium, uranium and thorium; and organics such as for example, oils, polychlorinated biphenyls, flue soot and others. Various techniques have been developed to remove specific contaminants. For instance, heavy metals are known to be found predominantly in the silt, humic or clay fraction of soil. Hence, they can be removed by size separation such as tiltable tables, concurrent flow in a mineral jig and chemical techniques such as the use of precipitants. The radioactive compounds when originating as a spill can be removed to a large extent by leaching. Since these compounds are often also present in the finer particles, the most severely contaminated fraction can also be removed by countercurrent flow size separation. Organics can be removed by washing with surfactants, thermal treatment or biological processes.
Special problems develop when the different types of contaminants are present in soil. Generally, biological or thermal processes are more effective for removing organics than washing. However, toxic inorganics such as lead or chromium (+6), if present, deactivate biological systems due to their toxicity and cause air pollution problems for thermal processes. In addition, thermal processes may mobilize otherwise fixed contaminants in the treated soil.
Radioactive contamination (e.g., uranium, thorium radium, etc.) can be removed by soil washing. Soil washing provides a means to process soils having multiple contaminants. The washed soil is accessible to further biological or thermal treatment. Inorganic and radioactive compounds may be separated from organics for separate sale or disposal.
Many soil washing processes are available. Most use mining equipment to provide intimate soil/surfactant contact. U.S. Pat. No. 4,783,253 discloses a process for separating radioactive contaminants from soil using a concurrent flow of water to float away lighter uncontaminated particles from heavy contaminated particles. The slurry of lighter particles is dewatered using a spiral classifier, centrifuge, filter or the like. U.S. Pat. No. 4,783,263 is directed to a process for removing toxic or hazardous substances, in particular organics, from soils and the like by converting the material to a slurry adding surfactants and/or alkaline agents, and concentrating the toxic substance in the liquid phase preferably with a modifier in a froth flotation cell.
Some of the limitations of the currently used processes are that they are optimized for removing only one type of contaminant or for cleaning only one type of soil, they are geared to cleaning the larger particles while concentrating the fines in a fraction for later disposal, and they often use filtration for water removal which is a capital intensive operation with high operating costs.
Once the contaminants have been removed from the soil or other particulate material they must in turn be recovered for further processing, such as mining and/or smelting in the case of heavy metals, or disposal, for example, through mixing with a fixative material such as concrete. The ability to recover contaminants from the cleaning system is to a large extent dependent upon the method by which the contaminants were removed from the soil in the first instance. Mineral extraction in general and soil washing in particular often require the oxidation of the metals and sometimes the organic fraction of the soil for the removal of the metals. Radioactive metals are also included with heavy metals requiring oxidation since most radioactive materials are also inorganic heavy metals, such as uranium, thorium or radium.
Some typical oxidants include nitric acid, sodium hypochlorite and calcium hypochlorite. However, the use of nitric acid is generally not practical due to the fact that nitric acid is nonselective in its action, dissolving the rock matrix as well as oxidizing and dissolving the metal of interest, it is expensive, and results in nitrate-laden waste liquors which can present environmental hazards unless treated. Sodium hypochlorite is expensive to use because commercial solutions are supplied as a 15% liquid which increases the freight cost. Calcium hypochlorite introduces large amounts of calcium ion into the leachate solution when used in quantities sufficient to oxidize the metal, and the calcium ions can then precipitate if carbonate bleach liquors are used or if the leachate solution is left standing in contact with air. This calcium carbonate precipitate is difficult to handle and can clog processing equipment. In addition, if common soaps are used to remove organics, the high calcium ion content tends to precipitate some of the soap which requires use of additional soap.
There is a need therefore for an improved process and apparatus for treating particulate materials, such as soil and the like, contaminated with mixed wastes such as radioactive materials, organics and heavy metals.
There is a further need for such a process and apparatus which separates organic and inorganic contaminants thereby allowing for optimum disposal routes or post treatment strategies to be used on the concentrated contaminated fractions.
There is also a need for such a process and apparatus which produces a high solids content fines stream.
There is yet another need for such a process and apparatus which is not capital intensive, is economical to operate and can be made portable for on-site treatment.
There is a further need for a system that can effectively recover the contaminants once they have been removed from the soil, requiring a minimal amount of equipment, chemicals, and being portable to the job site, which further allows for the processing of recovered contaminants, such as metals, through mining and/or smelting operations, and allows for effective leach-resistant fixation of contaminants which are to be disposed.